Image synchronization of scanning wafer inspection system

ABSTRACT

An inspection system comprises a beam generator module for deflecting spots across scan portions of a specimen. The system also includes detection channels for sensing light emanating from a specimen in response to an incident beam directed towards such specimen and generating a detected image for each scan portion. The system comprises a synchronization system comprising clock generator modules for generating timing signals for deflectors of the beam generator module to scan the spots across the scan portions at a specified frequency and each of the detection channels to generate the corresponding detected image at a specified sampling rate. The timing signals are generated based on a common system clock and cause the deflectors to scan the spots and the detection channels to generate a detected image at a synchronized timing so as to minimize jitter between the scan portions in the response image.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims the benefit under 35U.S.C. §120 of U.S. application Ser. No. 13/898,736, filed May 21, 2013,titled “IMAGE SYNCHRONIZATION OF SCANNING WAFER INSPECTION SYSTEM”, byKai Cao et al., which claims priority under 35 U.S.C. §119 of prior U.S.Provisional Application No. 61/800,547, filed Mar. 15, 2013, titled“IMAGE SYNCHRONIZATION OF SCANNING WAFER INSPECTION SYSTEM” by Kai Cao,et al. Both applications are herein incorporated by reference in theirentireties for all purposes.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to inspection and metrologysystems. More specifically, it relates to scanning type systems forinspecting and measuring semiconductor wafers and other types ofpatterned samples.

BACKGROUND

Generally, the industry of semiconductor manufacturing involves highlycomplex techniques for fabricating integrating circuits usingsemiconductor materials which are layered and patterned onto asubstrate, such as silicon. Due to the large scale of circuitintegration and the decreasing size of semiconductor devices, thefabricated devices have become increasingly sensitive to defects. Thatis, defects which cause faults in the device are becoming increasinglysmaller. Each device needs to be fault free prior to shipment to the endusers or customers.

Various inspection systems are used within the semiconductor industry todetect defects on a semiconductor reticle or wafer. Some conventionaloptical inspection tools locate defects on patterned wafers by scanningthe surface of the wafer with a tightly focused laser spot and measuringthe amount of light scattered by the illuminated spot on the wafer.Dissimilarities in the scattering intensity between similar locations inadjacent dies are recorded as potential defect sites.

Some conventional scanning systems include an illumination system one ormore incident beam sources for deflecting one or more beams across thewafer. The scanning system may specifically include an acousto-opticdeflectors (AOD's) and a mechanism for controlling the AOD's deflectioncharacteristics. For instance, a clock may be used to generate a “chirp”signal input to each AOD.

It would be beneficial to provide improved inspection systems havingdeflectors, such as AOD's.

SUMMARY

The following presents a simplified summary of the disclosure in orderto provide a basic understanding of certain embodiments of theinvention. This summary is not an extensive overview of the disclosureand it does not identify key/critical elements of the invention ordelineate the scope of the invention. Its sole purpose is to presentsome concepts disclosed herein in a simplified form as a prelude to themore detailed description that is presented later.

In one embodiment, a method of inspecting or measuring a specimen usingan inspection system comprising a pre-scanner acousto-optic deflector(AOD), a chirp AOD, and a plurality of detection channels is disclosed.A common trigger clock is used to generate a chirp clock and input achirp frequency ramp signal into the chirp AOD based on the generatedchirp clock. In response to the chirp frequency ramp signal input to thechirp AOD, a chirp packet is propagated through the chirp AOD. Thecommon trigger clock is also used to generate a pre-scanner clock andinput a pre-scanner frequency ramp signal into the pre-scanner AOD basedon the generated pre-scanner clock, and the pre-scanner AOD receives anddeflects an incident beam onto the propagating chirp packet in the chirpAOD, causing one or more spots to be scanned in a plurality of linesacross the specimen. The common trigger clock is also used to generatean acquisition clock for each detection channel and input a samplingfrequency signal into such detection channel based on the generatedacquisition clock for such detection channel. At each detection channel,light is detected from the specimen in response to one or more spotsscanned across the specimen and a detected image is detected that has asampling rate that is based on the sampling frequency signal.

In a specific implementation, the chirp clock has a same period as thecommon trigger clock. In a further aspect, the chirp frequency rampsignal is triggered off an edge of the chirp clock. In yet a furtheraspect, the chirp frequency ramp signal has a period that is equal tohalf a period of the chirp clock and the common trigger clock. Inanother embodiment, a period of the common trigger clock is selectedbased on a desired size of each spot. In a further aspect, the period ofthe common trigger clock is selected so that a period of the resultingchirp frequency ramp signal matches a relative stage movement thatcauses scanning of the one or more spots to move from a first set of oneor more scan lines to a second set of one or more scan lines. In yetanother example, the pre-scanner clock is delayed from the chirp clockby a time duration equal to a fill time of the chirp AOD minus a filltime of the pre-scanner AOD.

In another embodiment, the inspection system further comprises adiffractive element or mirror system for receiving a single spotdeflected from the chirp AOD and causing a plurality of spots to bescanned in a plurality of lines across the specimen. In anotherembodiment, each image acquisition clock of each detection channel istriggered after a fill time of the chirp AOD. In a further aspect, afrequency of each acquisition clock is adjusted based on a predefineddistortion amount of the corresponding detection channel. In yet afurther aspect, the frequency of each acquisition clock is adjusted pereach spot of the corresponding detection channel, wherein suchadjustment is based on a predefined distortion amount for such spot. Inanother feature, the acquisition clock and associated sampling frequencysignal for each detection channel is controlled so that sampling aplurality of locations on the specimen substantially accurately followsthe scanning of the one or more spots at they traverse along theplurality of lines. In yet another feature, the acquisition clock andassociated sampling frequency signal for each spot of each detectionchannel is controlled so that sampling a plurality of locations on thespecimen follows the scanning of such spot at it scans along a pluralityof lines. In another embodiment, the method includes analyzing thedetected images generated by the detection channels to detect defects onsuch specimen.

In an alternative embodiment, the invention pertains to a system forinspecting or measuring a specimen. This system comprises a beamgenerator module for deflecting one or more spots across a plurality ofscan portions of the specimen, and the scan portions include one or morefirst scan portions and one or more next scan portions that are scannedafter the one or more first scan portions. The system also includes oneor more detection channels for sensing light emanating from a specimenin response to an incident beam directed towards such specimen andgenerating a detected image for each scan portion as the incident beamis scanned over such scan portions. The system further comprises asynchronization system comprising a plurality of clock generator modulesfor generating a plurality of timing signals for one or more deflectorsof the beam generator module to scan the one or more spots across thescan portions at a specified frequency and for each of the detectionchannels to generate the corresponding detected image at a specifiedsampling rate. The timing signals are generated based on a common systemclock and cause the one or more deflectors to scan the one or more spotsand the detection channels to generate a detected image at asynchronized timing so as to minimize jitter between the scan portionsin the response image

In a specific embodiment, the clock generator modules comprise aplurality of direct digital synthesizers (DDS's) for generating ascanning clock for each of the one or more deflectors and generating asampling rate for each detection channel, wherein the synchronizationsystem further comprises a synchronization signal clock driver fordetermining a timing for each DDS module. In specific implementations,the beam generator module comprises a pre-scanner AOD and a chirp AOD,and the synchronization system is configured to perform one or more ofthe above-described operations.

These and other aspects of the invention are described further belowwith reference to the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of an example optical system inwhich embodiments of the present invention may be implemented.

FIG. 2 is a diagrammatic representation of scanning an illumination spotacross a sample and detecting light that is emitted from such sample inresponse to such illumination spot.

FIG. 3 is a diagrammatic representation of illumination optics of aninspection system that includes a pre-scanner acousto-optic deflector(AOD) and a chirp AOD.

FIG. 4 is a diagrammatic representation of scan timing associated with apre-scanner AOD and chirp AOD.

FIG. 5 is a diagrammatic representation of a synchronization system inaccordance with one embodiment of the present invention.

FIG. 6 is a flow chart illustrating a procedure for synchronizingscanning and image acquisition timing in accordance with a specificimplementation of the present invention.

FIG. 7 is a timing diagram illustrating synchronization of scanning andimage acquisition timing in accordance with a specific implementation ofthe present invention.

FIG. 8 show graphs for position distortion as a function of spotposition and image sampling frequency selection as a function of spotposition in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Thepresent invention may be practiced without some or all of these specificdetails. In other instances, well known process operations have not beendescribed in detail to not unnecessarily obscure the present invention.While the invention will be described in conjunction with the specificembodiments, it will be understood that it is not intended to limit theinvention to the embodiments.

In general, certain embodiments of the present invention employmechanisms for controlling the timing of an inspection system'sillumination scan rate and image (or signal) acquisition sampling rate.These timing mechanisms may be integrated into any suitable type ofinspection system that has an illumination system for scanning one ormore spots across a sample and one or more image acquisition channelsfor generating one or more detected signals or images based on detectedlight emitted from the sample in response to the one or moreillumination spots that are scanning across such sample. The inspectionsystem may include any suitable number and type of deflector anddetection modules or channels. In a specific optical scanning inspectiontool, an acousto-optic deflector (AOD) based scanning system issynchronized with the image acquisition system, resulting in an accurateimage with minimum line jitter. The optics distortion also can becorrected by changing the frequency of image acquisition's ADC clockswith direct digital synthesizer (DDS) technology. Timing mechanismembodiments of the present invention may be applied to other types ofdeflector types, such as galvanometer-driven mirrors, etc.

Prior to describing specific timing mechanisms, a general inspectionsystem will first be described. Although this system is described ashaving an illumination system for generating a single illumination spotand three detection channels, timing mechanisms of the present inventionmay be integrated with a system that generates multiple scanning spotsand/or has multiple detection channels for detecting light from anysuitable angle and any number of scanning spots.

FIG. 1 is a diagrammatic representation of an example optical system 100in which embodiments of the present invention may be implemented. Theoptical system 100 includes any suitable number of detectors orcollection channels for detecting light emitted from a sample, such as asemiconductor wafer surface. The detectors or collection channels may bearranged in any suitable position, and such arrangement depends on theparticular requirements of the inspection application. The illustratedembodiment uses two groups of two collector channels, 110 a-b and 111a-b, disposed symmetrically about the wafer surface 112 so that eachcollector channel within a pair is located at the same azimuthal angleon opposite sides of the scan line. These azimuthal collector channelsdetect scattered light. The output from the collector channels may thenbe sent to controller 101 for data analysis and/or image generation. Thedata from the channels may be compared by performing various algorithmsand logical operations, e.g., OR, AND and XOR.

The optical system also includes a beam generator (e.g., components 113,115, 116, and 117) for generating an incident beam and directing ittowards a sample. As shown in FIG. 1, a light source 113, typically alaser, emits a beam 114. Beam 114 is directed towards pre-deflectoroptics 115, which may include a half wave-plate, a spatial filter andseveral cylindrical lenses, in order to produce an elliptical beam witha desired polarization that is compatible with the deflector module 116.The pre-deflector optics 115 may be configured to expand the beam 114 toobtain the appropriate numerical aperture. The post-deflector optics 117may include several cylindrical lenses and an air slit. Finally, thebeam 114 may be brought into focus on the wafer surface 112 and scannedalong a particular direction, in the plane of the wafer surface 112,perpendicular to the optical axis of the beam 114. The type of deflectoremployed in the apparatus is application dependent. In one embodiment,deflector 116 includes one or more Acousto-Optic Deflectors (AODs).

The wafer surface 112 may be smooth 118 or patterned 119. In addition tothe collector channels 110 a-b and 111 a-b, described above, detectorchannels may be provided which include a reflectivity/auto-positionchannel 120, and a normal collector channel 121. each of which arediscussed more fully below.

The wavelength of the beam 114 depends on the particular requirements ofthe application. In the illustrated embodiment, the beam 114 has awavelength of about 488 nm. Beam 114 can be produced by any suitablelight source, such as an Argon ion laser. The optical axis 148 of thebeam 114 is directed onto the wafer surface 112 at an angle θ. Thisangle θ is preferably in the range of 55-85 degrees with respect to thenormal to the wafer surface 112, depending on the particularapplication. The scanning mechanism includes the deflector 116 and thetranslation or sample stage 124, upon which the wafer or sample rests.The position of the wafer on the stage 124 is maintained in anyconvenient manner, e.g., via vacuum suction. The stage 124 can move topartition the surface 112 into striped regions or scan lines, shown as125, 126 and 127 with the deflector 116 moving the beam across the widthof the striped regions.

Each illumination optics column may be moved with respect to the stageand/or the stage moved relative to each collection channel, includingone or more detectors or cameras, by any suitable mechanism so as toscan the sample. For example, a motor mechanism may be utilized to movethe stage or any other component of the system. Each motor mechanism maybe formed from a screw drive and stepper motor, linear drive withfeedback position, or band actuator and stepper motor, by way ofexamples.

Light scattered from the wafer surface 112 can be detected by aplurality of detectors, including collector channels 110 a-b and 111a-b. The collector channels can be arranged to collect light over afixed solid angle, dependent upon, inter alia, the elevational andazimuthal angle of the channel. The optical axis of each collectionchannel is positioned at an angle of elevation ψ in the range of 0 to 90degrees with respect to the normal to the surface 112. Collectorchannels 110 a and 110 b can be symmetrically positioned at the sameazimuthal angle with respect to beam 114, on opposite sides of the scanline. Collector channels 110 a and 110 b are positioned, with respect tothe beam 114, at an azimuthal angle ψ₁ in the range of about 75 to about105 degrees to collect laterally scattered light. Laterally scatteredlight is defined as light scattered at azimuthal angles in the range ofabout 75 to about 105 degrees, with respect to beam 114. Similar tocollector channels 110 a and 110 b, channels 111 a and 111 b can bepositioned on opposite sides of the scan line at the same azimuthalangle. However, the azimuthal angles ψ₂ of channels 111 a and 111 b arein the range of 30 to 60 degrees, to collect forwardly scattered light.Forwardly scattered light is defined as light scattered at azimuthalangles in the range of 30 to 60 degrees. Those of ordinary skill in theart will readily recognize that the number and location of the collectorchannels and/or their collection solid angle may be changed in variousalternative embodiments without departing from the scope of theinvention.

The bright field reflectivity/autoposition channel 120, can bepositioned in front of the beam 114 to collect specularly reflectedlight. The bright field signal derived from this channel carriesinformation concerning the pattern, local variations in reflectivity andheight. This channel is sensitive to detecting various defects on asurface. For example, the bright field signal is sensitive torepresenting film thickness variations, discoloration, stains and localchanges in dielectric constant. The bright field signal is also used toproduce an error height signal, corresponding to a variation in waferheight, which is fed to a z-stage to adjust the height accordingly.Finally, the bright field signal can be used to construct a reflectivitymap of the surface. In one embodiment, this channel is basically anunfolded Type I confocal microscope operating in reflection mode. It isconsidered unfolded because the illuminating beam and reflected beams,here, are not collinear, as compared with a typical reflection confocalmicroscope in which the illuminating and reflected beams are collinear.

The normal collector channel collects light over a fixed solid angleover a region which is approximately perpendicular to the plane of thewafer. Other than the collection solid angle, the normal collectorimplementation may be similar to the other collector channels 110 ab and111 ab. The normal collector may be used to collect scattered light fromthe intentional patterns on the wafer, as well as to detect defectswhich scatter light in an upwards direction. Signals collected from theintentional patterns may be used to facilitate the alignment andregistration of the wafer pattern to the coordinate system of themechanical stage in the instrument.

One or more of the collector channels may include mechanism forincreasing the dynamic range of detected output signals. Preferably,these mechanisms for increasing dynamic range are provided withincollector channels 110 ab, 111 ab and 121. In general terms, a highdynamic range collector includes a light sensor, such as aphotomultiplier tube (PMT), for generating a signal from detectedphotons and an analog to digital converter (ADC) for converting thelight signal to a digital light signal. Suitable PMT's include acircular cage type PMT, metal-channel photomultiplier, etc. Of course,other suitable mechanism may be used for sensing light and converting ananalog signal into a digital signal.

The grazing angle of the beam 114 may produce an elliptical spot on thewafer surface 112, having a major axis perpendicular to the scan line.The deflectors 116 scan the spot across a short scan line equal inlength to the length of scan line 125 to produce reflected and scatteredlight. The spot may be scanned in a first direction, as the stage 124moves the wafer perpendicular to the scan line. This results in the spotmoving along scan line 125.

In one implementation, the illumination beam 114 is raster scanned alongmultiple scan lines 125, 126, and 127 one at a time. For example, afirst scan line 125 has an effective start location and the spot movesfrom left to right along such first scan line until the beam reaches theborder of the first scan line. Upon reaching the border of scan line125, the spot moves relative to the stage 124 perpendicular to the scandirection and the spot then has a new start position for a new secondscan line 126. The spot then moves along this second scan line 126parallel to the first scan line. The deflector 116 continues to scan thespot in this fashion along the entire length of the second scan line126. Upon completion of the second scan line 126, the stage 124 movesrelative to the wafer to permit the scanning of the adjacent third scanline 127. The spot moves along the third scan line 127 in a directionopposite to that when scanning the second scan line 126, thereby forminga serpentine scan.

FIG. 2 is a diagrammatic representation of scanning an illumination spotacross a scan line of wafer 201 and detecting light that is emitted fromsuch wafer in response to such illumination spot. As shown, illuminationbeam 202 is scanned in direction 214 across wafer 201. As such beam 202moves, illumination spot is scanned across a scan line of such wafer.For instance, the scanning illumination beam 202 causes spot 203 a at afirst time to be formed at position 204 a and then causes spot 203 b ata second subsequent time to be formed at position 204 b on the specimen201. Thus, a spot moves in direction 214.

As each illumination spot is moved along a scan line, correspondinglight 206 may be detected by a sensor, such as photo-multiplier tube(PMT) 208. The PMT generates and outputs a signal based on the detectedlight to an analog-to-digital (ADC) 210. As the PMT detects andcontinuously outputs a detected signal corresponding to different waferpositions along the scan line, the ADC samples at a specified samplingrate the detected signal and produces a digital representation of eachdetected signal sample interval. That is, the ADC 210 samples andconverts portions of the detected signal at particular time intervals.Each sampling time interval corresponds to particular positions on thewafer. The digital representation may then be used to generate an image.

To prevent line jitter occurring in an image produced from the sampledsignal, the timing of the ADC 210 is controlled such that the sampledportions of the detected signal correspond to each spot position on thewafer 201. For instance, a first sampling time interval corresponds tofirst spot 203 a having a center position 204 a and edge positions 212 aand 212 b. The second sampling time would then correspond to a secondspot 203 b at center position 204 b. Thus, the ADC's sampling time isselected to follow the timing of the spot as it moves across each scanline. The ADC also samples each end pixel or spot of each scan line atthe correct position on the wafer so that the resulting image lines arenot skewed with respect to each other. For instance, the sampling rateinput to the ADC may be selected to prevent the image sampling of eachscan line from starting at an early or delayed position that correspondsto an edge of the first spot of a scan line (e.g., 212 a or 112 b), butrather starting at the center position (e.g., 204 a) of the first pixelof the scan line. The sampling may result in different spot and pixelrelationships. For instance, a spot can correspond to one pixel ormultiple pixels.

In slower scan systems, precise synchronization between the samplingtiming of the image acquisition and the scan timing may not be asignificant issue. For instance, a 100 MHz system may correspond to a 10ns width for each pixel, and a sampling error of 2 ns may correspond toonly ⅕ a pixel. However, a faster system, such as a 5 GHz, maycorrespond to a 200 ps pixel width, and a 2 ns sampling error wouldcorrespond to a 10 pixel error, which would cause the acquired image toappear significantly skewed. Accordingly, certain embodiments of thepresent invention provide timing control such that the image acquisitionsystem operates at an imaging rate that is synchronized with the scanrate. The resulting image is not skewed.

In some inspection systems, a pre-scanner AOD and a chirp AOD areutilized to scan a spot across each line on the sample under test. FIG.3 is a diagrammatic representation of illumination optics of aninspection system that includes a pre-scanner acousto-optic deflector(AOD) 344 and a chirp AOD 354. The pre-scanner AOD 344 and chirp AOD 354may be made of a solid medium that includes, but is not limited to, acrystal material such as TeO₂, quartz, fused silica, sapphire, anotherglassy material, or any other appropriate material known in the art.

A sound transducer 346 may be coupled with a solid medium surface of thepre-scanner AOD 344. Transducer 346 may be configured to generate adrive signal which fills pre-scanner AOD 344 with a sound wave whosefrequency varies slowly compared to the propagation time of the soundwave through pre-scanner AOD 344. By varying the frequency of the soundwave in pre-scanner AOD 344, the deflected beam may be scanned fromlocation 362 to location 364.

The system may also include one or more lens 352. Lens 352 may beconfigured to expand the beam and convert the small angular scan frompre-scanner AOD 344 into a long linear scan at chirp AOD 354. Forinstance, lens 352 receives a beam at a first position 362 and transmitsbeam 363. Likewise, lens 352 receives a beam at a last position 364 andtransmits beam 365. The lens 352 may also be configured to manipulatethe received beam in any number and type of manners. For example, thelens may include a telescope, a relay lens, a focusing lens, anobjective lens, a mirror, or any other appropriate optical componentknown in the art.

Chirp AOD 354 may be operated in chirp mode. Transducer 358 attached orcoupled to chirp AOD 354 may be configured to generate a drive signal,which produces a chirp packet that propagates over a width of chirp AOD354 from position 359 to position 360. The chirp packet generally takesa finite time to form, determined by the desired width of the chirppacket and the acoustic velocity in the AOD. This chirp packet creationtime may be referred to herein as a “fill time.”

A chirp packet propagating through chirp AOD 354 may be configured tofunction as a traveling lens to focus the scanning beam into a spot. Thewidth of a chirp packet may be approximately equal to the size of thereceived light beams, e.g., 363 and 365, which is much less than thewidth of chirp AOD 354. Alternatively, the chirp AOD 354 may be causedto contain multiple chirp packets at the same time. Each chirp packetmay be substantially shorter than the width of the AOD.

A chirp packet propagating through an AOD's solid medium may have afrequency in the ultrasonic range. The chirp packet propagating throughsuch solid medium may alter a property of the solid medium, such as alattice structure of the crystal or a refractive index. In this manner,a light beam incident on the solid medium of the AOD may propagatethrough the solid medium and may be diffracted by a portion of thecrystal lattice altered by the ultrasonic chirp packet as it propagatesthrough the crystal. As a result, a portion of light exiting an AOD'ssolid medium may include a deflected beam. A portion of light exitingthe solid medium of an AOD, however, may also include one or moresubstantially undeflected beams. A chirp packet may contain multiplefrequencies that change linearly from the start of the packet to the endof the chirp packet commonly referred to as a “frequency ramp.”

An angle at which an incident beam may be deflected by an AOD may dependonly upon relative wavelengths of light and ultrasound waves inside theAOD. In this manner, an angle of deflection of a beam exiting an AOD maybe determined and may be controlled by a wavelength of light incidentupon the AOD and a wavelength of an ultrasonic sound wave induced insidethe solid medium of the AOD.

For the case of the chirp mode where the drive frequency changeslinearly over a chirp packet, the incident beam is diffracted atdifferent angles proportional to the frequency in the chirp packet. Byramping the frequencies from low to high and as illustrated in FIG. 3, aparticular portion 322 of the chirp packet 360 may have a higherfrequency than another portion 320 of the same chirp packet 360. Becauseportion 322 has a higher frequency, it diffracts a portion of incidentlight beam 365 through a steeper angle as shown by diffracted beam 316.Because portion 320 has a relatively lower frequency, it diffracts aportion of incident light beam 365 through a more shallow angle as shownby diffracted light beam 318. In this manner, a chirp packet can be usedto focus beam in the plane shown as scan line 330.

An AOD configured in a chirp mode may be restricted to having abandwidth, or a range of frequencies, of less than approximately 1octave. Such bandwidth limitations may minimize, or may substantiallyeliminate, secondary beams of light deflected by the AOD from scanningthe surface of a specimen at the same time as the primary beam of lightdeflected by the AOD. Such an AOD configuration, however, will producechief rays (e.g., 316 and 318) that will not be perpendicular to scanline 330 generated by AOD 354.

A light source (not shown) and pre-scanner AOD may be configured todirect light to illuminate a single chirp packet as it propagatesthrough chirp AOD 354. In other applications, the light source andpre-scanner AOD may be configured to direct light across substantiallyan entire width of chirp AOD 354. Such a configuration of a light sourceand AOD may be referred to herein as a “flood mode” configuration. Inthis manner, light may be directed to first chirp packet and a secondchirp packet or any number of chirp packets along a width of the AODsubstantially simultaneously.

As chirp packet propagates through chirp AOD 354 away from transducer358 from position 359 to position 360 in direction 356, the chirp packetmay be attenuated in amplitude. Consequently, light focused onto a scanline by the chirp packet at position 359 may be brighter than lightfocused onto a scan line by a chirp packet at position 360. Thisnon-uniformity in scan line brightness may detrimentally affect theperformance of the inspection system or the matching of multiplesystems.

In order to compensate for attenuation of a chirp packet as itpropagates through chirp AOD 354, the brightness of the beamilluminating the chirp packet may be varied. This may be accomplished byvarying the amplitude of the drive signal applied to first pre-scannerAOD 344 by transducer 346. At the start of the beam sweep, pre-scannerAOD 344 may be driven with a lower amplitude signal, to produce a lessbright beam 362 which then illuminates chirp packet at position 359 neartransducer 358 in chirp AOD 354. At the end of the beam sweep,pre-scanner AOD 344 may be driven with a higher amplitude signal, toproduce a brighter beam 364 which then illuminates chirp packet atposition 360 at the end of chirp AOD 354. Amplitude modulation ofpre-scanner AOD 344 may thereby compensate for attenuation within chirpAOD 354, producing a final scan line with substantially uniformbrightness.

The brightness of a scan line produced by a system as described abovemay be calibrated by scanning a specimen of uniform reflectivity. Lightscattered from different positions along the final scan line may becollected and measured. The amplitude of the drive signal applied to thefirst AOD may then be modulated as needed to produce a scan line ofmeasured uniform brightness at the specimen. This calibration maycompensate not only for attenuation in the second AOD, but for any othernon-uniformities in the scanning system.

Each illumination column may also include one or more lens for receivingand manipulating the beams, e.g., 316 and 318, from the chirp AOD 354,so as to scan a spot across the specimen 327, e.g., in direction 314.The illumination system may also be configured to generate multiplespots on the sample. As shown, a diffractive element or mirror system302 may be configured to direct multiple spots to multiple illuminationcolumns (e.g., the illustrated column includes optics elements 336 and326). The inspection system may also include multiple detection channelsfor detecting light from different angles or spots.

A single illumination column (e.g., 336 and 326) for scanning a spotacross the sample (e.g., from position 328 a to 328 b in direction 314)is illustrated for simplification. As shown, the illumination column mayinclude relay lens 336. Relay lens 336 may be configured to collimatelight focused by AOD 354. Relay lens 336 may include any appropriatelens known in the art. Optical axis 335 of relay lens 336 may becentered on scan line 330 produced by AOD 354. Optical axis 335 may beparallel to the chief rays (non-secondary rays) of AOD 354.

The system may also include objective lens 326. Objective lens 326 maybe configured to focus the light collimated by relay lens 336 onto thefocal plane, which is parallel to the surface of the sample 327.Objective lens 326 may include any focusing lens known in the art. Theoptical axis 335 of relay lens 336 may be centered on scan line 330produced by AOD 354. In addition, optical axis 335 of relay lens 336 maybe perpendicular to scan line 330 produced by AOD 354. Optical axis 335of relay lens 336 may not be substantially parallel to chief ray(non-secondary) produced by AOD 354.

Optical axis 333 of objective lens 326 may be substantially de-centeredwith respect to optical axis 335 of relay lens 336. Optical axis 333 ofobjective lens 326 may be substantially parallel to optical axis 335 ofrelay lens 336. The pupil of the light collimated and formed by relaylens 336, however, may be substantially centered on objective lens 326.In addition, objective lens 326 may be substantially parallel to thefocal plane. In this manner, objective lens 326 may be substantiallycentered on the focal plane. As such, chief ray (non-secondary)deflected by AOD 354 may be relayed by this optical system at asubstantially perpendicular angle to the focal plane. Furthermore, thefocal plane may be substantially parallel to surface of specimen 327. Inthis manner, an angle at which the focal plane may be located withrespect to the surface of the specimen may be approximately 0 degrees.Therefore, field tilt associated with a chirp mode of an AOD may becorrected by a system in which the optical axis of an objective lens maybe offset from the optical axis of a relay lens.

In an alternative embodiment, optical axis 335 of relay lens 336 may becentered on scan line 330 produced by AOD 354. Chief rays produced byAOD 354 may not be substantially parallel to optical axis 338. Opticalaxis 338 of relay lens 336 may be perpendicular to scan line 330produced by AOD 354. Relay lens 336 may be configured to collimate lightdeflected and focused by the AOD as described in above embodiments. Assuch, in such an embodiment, light collimated by relay lens 336 may notbe centered on objective lens 326.

In an additional embodiment, the system may further include opticalmechanism, such as a prism assembly or system of mirrors positionedbetween the relay and the objective lenses. The system of mirrors orprism assembly may be configured to re-center a pupil of the lightcollimated by relay lens 336 onto objective lens 326.

The illumination system may also include additional optical components(not shown). For example, additional optical components may include, butmay not be limited to, beam splitters, quarter wave plates, polarizerssuch as linear and circular polarizers, rotating polarizers, rotatinganalyzers, collimators, focusing lenses, mirrors, dichroic mirrors,partially transmissive mirrors, filters such as spectral or polarizingfilters, spatial filters, reflectors, and modulators. Each of theseadditional optical components may be disposed within the system or maybe coupled to any of the components of the system as described herein.

Certain embodiments of the present invention include mechanisms forsynchronizing the timing of the illumination and image acquisition so asto minimize line jitter in the acquired image. The XY position in theacquired image and scan line is at least partially controlled by timingsignals input to the illumination deflectors (e.g., pre-scanner andchirp AOD) and the image acquisition sampling components (e.g., imageacquisition ADC's).

The scan timing of a high speed inspection system depends on thedeflector timing. FIG. 4 is a diagrammatic representation of scan timingassociated with a pre-scanner AOD and chirp AOD. As shown, a pre-scannerfill time (T_(psf)) is associated with the pre-scanner 344. The T_(psf)corresponds to the amount of time it takes to fill the pre-scanner AODwith a sound wave. A chirp fill time (T_(cf)) is also associated withchirp AOD 354, and this chirp fill time T_(cf) corresponds to the amountof time to fill the chirp AOD with a chirp sound wave (or the time toproduce a chirp packet in the chirp AOD). Time T_(img) corresponds tothe amount of time the chirp packet takes to scan across the chirp AODand also corresponds to the time duration during which a spot is scannedacross a scan line of the specimen. T_(img) also represents the timeduring which an image is to be acquired from the specimen in response tothe scanned beam.

In certain embodiments, a common clock, such as a fast 2.5 GHz clock, isused to control both the scanning and image acquisition systems of theinspection tool. In a specific example, this fast clock is used togenerate 100's of MHz clocks for the AOD and image acquisition system'sADC's. The ADC sampling clocks can be adjusted dynamically to compensatefor optical distortion. Optical distortion can be correctedindependently for each spot in a multiple scan spot system.

FIG. 5 is a diagrammatic representation of a synchronization system 500in accordance with one embodiment of the present invention. Thesynchronization system 500 may include any number and type of clockgenerator modules for generating a clock for each scan and eachdetection channel. For example, a frequency synthesizer for creating awaveform from a fixed-frequency reference clock may take the form of adirect digital synthesizer (DDS), etc. In the illustratedimplementation, the synchronization system 500 includes a DDS forgenerating a clock at a selected frequency for each scanner and imagechannel. As shown, DDS 512 a-512 e are configured to dynamicallygenerate clocks for the multiple detection channels of image acquisitionsystem 516 (e.g., the ADC sampling clocks). The system may also includeDDS 510 for generating a clock for the pre-scanner AOD (e.g., ofpre-scanner and chirp AOD module 514) and a DDS 508 for generating aclock for the chirp AOD (e.g., of pre-scanner and chirp AOD module 514).

Each DDS module may receive a system clock, which is distributed byclock driver 504 from a system clock generator module 502, and asynchronization signal sync_in signal from a synchronization signalclock driver 506. For instance, the system clock generator module 502may generate a 2.5 or 5 GHz system clock and the clock driver 504 thendistributes this system clock to each DDS.

Each DDS may include any number of components for generating a selectedclock for each AOD and ADC. For instance, each DDS may include anumerically (e.g., digitally) controlled frequency register. An inputsystem clock provides a stable time base and provides the clock to theDDS, which produces a discrete-time, quantized version of the desiredoutput with the period controlled by input received into the frequencyselection register (from the synchronization signal clock driver 506).Each DDS may also include hardware and/or software for dynamic phase anddynamic frequency control.

A digital control module 501 may be configured to receive an externaltrigger clock and the locally generated system clock Sync_out from thesystem clock generator 502. The digital control module 501 may also beconfigured to command the timing sequence for the different AOD andimage acquisition clocks (e.g., as shown in FIG. 7) with such timingbeing synchronized to the local system clock. The synchronization signalclock driver 506 can also be configured to control the timing of the DDSmodules and their corresponding scanner and image acquisition channels,for example, by locking the timing of the DDS modules to be insynchronization with respect to each other. The synchronization signalclock driver 506 may receive the system clock (e.g. sync out) andgenerate independent clocks or triggers (e.g., sync_in) for each DDSmodule. The synchronization signal clock driver 506 may be formed fromany suitable combination of hardware and/or software. For example,synchronization signal clock driver 506 may include one or more FPGA's(field programmable gate arrays), ASIC's (application specificintegrated circuits), other logic devices, etc.

Any suitable technique may be utilized to synchronize the scanning andimage acquisition timing. FIG. 6 is a flow chart illustrating aprocedure 600 for synchronizing scanning and image acquisition timing inaccordance with a specific implementation of the present invention.Initially, a common system clock may be used to generate a chirp clockand input a chirp frequency ramp into the chirp AOD (of the inspectiontool's scanning system) based on the generated chirp clock in operation602. In response to the chirp frequency ramp that is input to the chirpAOD, a chirp packet propagates through the chirp AOD in operation 604.This chirp packet has a frequency that depends on the frequency of thechirp frequency ramp, which depends on the frequency of the chirp clock.The timing of both the chirp frequency ramp and the chirp clock dependon the timing of a trigger signal that is based on the system clock.

FIG. 7 is a timing diagram illustrating synchronization of scanning andimage acquisition timing in accordance with a specific implementation ofthe present invention. As shown, a trigger signal (or clock) havingperiod T_(line) is used to generate chirp clock “Chirp_digital”. Forexample, the digital chirp clock has the same period T_(line) as thetrigger clock. This digital chirp clock is also used to generate thefrequency ramp signal (e.g., Chirp_in_AOD) that is input to the chirpAOD. For instance, a frequency ramp signal is triggered off each risingedge of the digital chirp clock “Chirp_digital” (following a small chirpdead time T_(dp)). As shown, a first frequency ramp 706 a is triggeredby a first edge 708 a of the trigger clock, and a second frequency ramp706 b is triggered off the subsequent second edge 708 b of the triggerclock. As shown, the frequency input to the chirp AOD ramps down from afirst frequency f_(c1) to a second frequency f_(c2). The frequency rampduration is based on a half cycle (e.g., active high) of the digitalchirp clock “Chirp_digital.”

The width of an AOD's chirp packet is generally based on thecorresponding width of the input frequency ramp. As described herein,the chirp packet focuses beams received from the pre-scanner into a spotonto a scan line, and such spot moves along with the chirp packet as itpropagates through the chirp AOD. Since the chirp packet forms a spotand the chirp packet is formed from the chirp frequency ramp, which isbased on the trigger clock, the period of the trigger clock is directlyrelated to the chirp packet size and resulting spot size. Accordingly,the trigger clock's period may be selected based on the desired spotsize.

The chirp frequency ramp signal may cause chirp packets to propagate oneat a time through the chirp AOD so as to cause a spot to scan along ascan line of the specimen. For example, a first frequency ramp (706 a)may be input into a first end of a chirp AOD and cause a first chirppacket to propagate from this first end to a second end of the chirpAOD. As the first chirp packet reaches the second end of the chirp AOD,a second frequency ramp (706 b) may then cause a second chirp packet topropagate from the first to the second end of the AOD. When each chirppacket reaches the second end, which corresponds to the scan line end,the stage, upon which the specimen is placed, may also be moved relativeto the axis of the particular spot so that another line is then scannedin response to the second chirp packet propagation. The trigger clockperiod may also be selected so that a period of the resulting chirpfrequency ramp signal matches the relative stage movement from scan lineto scan line. Likewise, the width of the AOD may be selected so thateach chirp packet results in a spot being scanned across aligned scanlines. For instance, a first chirp packet results in a spot movingbetween two x positions of a first scan line (y1), and a second chirppacket results in a spot moving from two x positions of a second scanline (y2) so that each scan line has the same delta x positions.

Referring back to FIG. 6, the trigger clock is also used to generate apre-scanner clock and input a pre-scanner frequency ramp to thepre-scanner AOD based on such pre-scanner clock in operation 606. Thetiming of the pre-scanner clock is generally based on the fill time ofthe chirp AOD and the pre-scanner AOD. As shown in FIG. 7, a pre-scanclock “Pre-Scan_digital” is started after time T_(dly), which is equalto the chirp AOD fill time T_(cf) minus the pre-scanner AOD fill timeT_(psf). The pre-scanner frequency ramp “Pre-scan_in_AOD”, which isinput to the pre-scanner AOD, is triggered off the rising edge of thepre-scanner clock “Pre-Scan_digital” (following a small pre-scanner deadtime T_(dp)). As shown, the frequency input to the pre-scanner AOD rampsup from a first frequency f_(p1) to a second frequency f_(p2).

At the pre-scanner AOD, an incident beam is received and deflected ontothe propagating chirp packet in the chirp AOD, causing one or more spotsto be scanned in a plurality of scan lines across the sample inoperation 608. For instance, a plurality of spots can be formed from asingle scanned spot that is output from the chirp AOD, and thesemultiple spots are directed to scan onto the sample. Alternatively,multiple deflector systems (e.g., multiple pre-scanner and chirp AOD's)may be used to generate multiple spots on the sample. Regardless of thenumber of spots per pre-scanner period, multiple scan lines may bescanned by moving the sample stage relative to the scanning spots.

The common trigger clock may also be used to generate an acquisitionclock for each detection channel and input a sampling frequency clock(e.g., IA_ADC clock) into such detection channel based on such generatedacquisition clock in operation 610. For each detection channel, light isdetected from the sample in response to the one or more spots scannedacross the sample and a detected signal or image is generated. Eachchannel may detect light from a particular angle and/or from aparticular spot. For each detection channel, light is detected from thesample in response to one or more spots that are scanned across thechannel and a detected image (or signal) is generated to have a samplingrate based on the generated acquisition clock and sampling frequencyclock in operation 612. The procedure may then end.

As shown in FIG. 7, an image acquisition clock “Image_Collect” maytriggered after the chirp fill time delay T_(cf). The image acquisitionclock has an image collection region that corresponds to when an imageis sampled for the corresponding scan line duration T_(img). Acorresponding sampling clock “IA_ADC” is input to the particulardetection channel based on the generated image acquisition clock forsuch particular acquisition clock. As shown, the sampling clock “IA_ADCclock” is triggered off the rising edge 702 of the acquisition clock“Image_Collect.” Timing portion 703 is an expanded view of area 704 ofthe Image_Collect and IA_ADC clock.

Each image acquisition and associated sampling clock may also beadjusted to compensate for optical distortion in the respective channel.For instance, it may first be determined how much distortion in positionis present for each scan spot. A reference sample with known structuresat known positions may be inspected to obtain a test image. The testimage can then be compared to a reference image that is simulatedwithout optical distortion. For instance, the design data for areference wafer may be used to simulate a reference image of variousspots that are produced by ideal optics having no distortion. For eachspot, the difference between positions of the test image structures maythen be compared with the positions of the reference structures toobtain a distortion amount. Different sampling frequencies can then beapplied to the test sample to obtain an optimum frequency for samplingeach spot so that the distortion is corrected for such spot. The optimumfrequency to correct each spot's distortion may then be stored for lateruse by the sampling clock generator. For example, the image acquisitionclock (and resulting sampling clock) for each spot may then be adjustedby a frequency amount that is selected to correct the distortion forsuch spot.

FIG. 8 show graphs for position distortion as a function of spotposition and image sampling frequency selection as a function of spotposition in accordance with one embodiment of the present invention.Graph 802 illustrates distortion as a function of the relative positionof 9 spots. Graph 804 illustrates the frequency for correcting thedistortion for each relative position of the 9 spots. For instance, theimage acquisition clock for the spot at position 1 can be set to about493.5 MHz, while the acquisition clock for the spot at position 2 may beset at about 494.5 MHz.

In general, controlling the sampling rate controls the locations thatare sampled on the sample as each spot is scanned across such sample.The sampled locations can be controlled so that the sampled locationsfollow the spots as they scan across scan lines of the sample.Additionally, the sampling of each spot can be adjusted so that thesampled location precisely follows such spot as it traverses from lineto line so as to minimize line jitter. The sampling of each spot can beadjusted so that the scanned location compensates for distortion inoptics for such spot (e.g., illumination and/or collection optics).

The detected images (or signals) may then be analyzed to determinewhether defects are present on the sample. For example, the intensityvalues from a target die are compared to the intensity values from acorresponding portion of a reference die (or generated from a designdatabase), where a significant intensity difference may be defined as adefect. These inspection systems may implement any suitable inspectiontechnology, along with the novel image synchronization mechanismsdescribed further below. By way of examples, brightfield and/ordarkfield optical inspection mechanisms may be utilized. The mechanismsof the present invention may also be implemented within a scanningelectron microscopy system.

Each detected image may also be input to a defect (e.g., image)processor (e.g., 101). Defect processor may include mechanisms forprocessing the received data, such as buffering, compressing, packing,filtering noise, generating images based on the input signal, analyzingimages to detect defects on the sample, etc. The majority of defects maybe found by detecting contrast, defined as the ratio of the intensitiesin the target and reference dies, rather than by threshold, which isdefined as the difference between the intensities.

The inspection techniques described herein may be implemented on variousspecially configured inspection or metrology systems, such as the oneschematically illustrated in FIG. 1. In certain embodiments, a systemfor inspecting or measuring a specimen includes various controllercomponents for implementing the techniques described herein. Thecontroller may be implemented by any suitable combination of hardwareand/or software, such as a processor, memory, programmable device orfield programmable gate array (FPGA), etc.

The inspection system may be associated with a computer system that isconfigured (e.g., with programming instructions) to provide a userinterface (e.g., on a computer screen) for displaying resultantinspection characteristics. The computer system may also include one ormore input devices (e.g., a keyboard, mouse, joystick) for providinguser input, such as changing detection parameters. In certainembodiments, the computer system is configured to carry out inspectiontechniques in conjunction with other inspection components, such ascontroller 101, detailed herein. The computer system typically has oneor more processors coupled to input/output ports, and one or morememories via appropriate buses or other communication mechanisms.

Because such information and program instructions may be implemented ona specially configured computer system, such a system includes programinstructions/computer code for performing various operations describedherein that can be stored on a computer readable media. Examples ofmachine-readable media include, but are not limited to, magnetic mediasuch as hard disks, floppy disks, and magnetic tape; optical media suchas CD-ROM disks; magneto-optical media such as optical disks; andhardware devices that are specially configured to store and performprogram instructions, such as read-only memory devices (ROM) and randomaccess memory (RAM). Examples of program instructions include bothmachine code, such as produced by a compiler, and files containinghigher level code that may be executed by the computer using aninterpreter.

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced within the scope of theappended claims. It should be noted that there are many alternative waysof implementing the processes, systems, and apparatus of the presentinvention. Accordingly, the present embodiments are to be considered asillustrative and not restrictive, and the invention is not to be limitedto the details given herein.

What is claimed is:
 1. A method of inspecting or measuring a specimenusing an inspection system comprising a beam generator module and aplurality of detection channels, the method comprising: using a commontrigger clock to generate one or more deflectors clocks that are inputto the beam generator module that receives and deflects an incident beamso as to cause one or more spots to be scanned in a plurality of linesacross the specimen; using the common trigger clock to generate anacquisition clock for each detection channel and input a samplingfrequency signal into such detection channel based on the generatedacquisition clock for such detection channel; and at each detectionchannel, detecting light from the specimen in response to the one ormore spots scanned across the specimen and generating a detected imagethat has a sampling rate that is based on the sampling frequency signal.2. The method of claim 1, wherein at least one of the one or moredeflector clocks has a same period as the common trigger clock.
 3. Themethod of claim 1, wherein a period of the common trigger clock isselected based on a desired size of each spot.
 4. The method of claim 1,wherein the beam generator module comprises a pre-scanner acousto-opticdeflector (AOD) and a chirp AOD, wherein the one or more deflectorclocks include a chirp clock for inputting a chirp frequency ramp signalinto the chirp AOI, wherein the one or more deflector clocks furtherinclude a pre-scanner clock for inputting a pre-scanner frequency rampsignal into the pre-scanner AOI, wherein a period of the common triggerclock is selected so that a period of the resulting chirp frequency rampsignal matches a relative stage movement that causes scanning of the oneor more spots to move from a first set of one or more scan lines to asecond set of one or more scan lines.
 5. The method of claim 4, whereinthe pre-scanner clock is delayed from the chirp clock by a time durationequal to a fill time of a chirp AOD propagating through the chirp AODminus a fill time of the pre-scanner AOD.
 6. The method of claim 4,wherein the inspection system further comprises a diffractive element ormirror system for receiving a single spot deflected from the chirp AODand causing a plurality of spots to be scanned in a plurality of linesacross the specimen.
 7. The method of claim 1, wherein the acquisitionclock and associated sampling frequency signal for each detectionchannel is controlled so that sampling a plurality of locations on thespecimen substantially accurately follows the scanning of the one ormore spots as they traverse along the plurality of lines.
 8. The methodof claim 1, wherein the acquisition clock and associated samplingfrequency signal for each spot of each detection channel is controlledso that sampling a plurality of locations on the specimen follows thescanning of such spot at it scans along a plurality of lines.
 9. Ansystem for inspecting or measuring a specimen, comprising: a beamgenerator module for scanning one or more incident spots across aplurality of scan portions of the specimen, wherein the scan portionsinclude one or more first scan portions and one or more next scanportions that are scanned after the one or more first scan portions; oneor more detection channels for sensing light emanating from the specimenin response to the one or more incident spots scanned across the scanportions and generating a detected image for each scan portion as anincident beam is scanned over such scan portions; and a synchronizationsystem comprising a plurality of clock generator modules for receiving acommon clock and generating one or more deflectors clocks that are inputto the beam generator module so as to cause the one or more spots to bescanned across the scan portions and generating an acquisition clock forinputting a sampling frequency signal into each detection channel basedon the generated one or more deflector clocks to generate a detectedimage at a synchronized timing so as to minimize jitter between the scanportions in the detected image, wherein the one or more deflector clocksand the acquisition clock are generated based on the common systemclock.
 10. The system of claim 9, wherein the clock generator modulescomprise a plurality of direct digital synthesizers (DDS's) forgenerating a scanning clock for the beam generator module and generatinga sampling rate for each detection channel, wherein the synchronizationsystem further comprises a synchronization signal clock driver fordetermining a timing for each DDS module.
 11. The system of claim 9,wherein the beam generator module comprises a pre-scanner AOD and achirp AOD and wherein the synchronization system is configured toperform the following operations: using the common trigger clock togenerate a chirp clock and input a chirp frequency ramp signal into thechirp AOD based on the generated chirp clock; in response to the chirpfrequency ramp signal input to the chirp AOD, propagating a chirp packetthrough the chirp AOD; using the common trigger clock to generate apre-scanner clock and input a pre-scanner frequency ramp signal into thepre-scanner AOD based on the generated pre-scanner clock, wherein thepre-scanner AOD receives and deflects an incident beam onto thepropagating chirp packet in the chirp AOD, causing the one or moreincident spots to be scanned in a plurality of lines across thespecimen; using the common trigger clock to generate the acquisitionclock for each detection channel and input a sampling frequency signalinto such detection channel based on the generated acquisition clock forsuch detection channel; and at each detection channel, causing light tobe detected from the specimen in response to the one or more incidentspots scanned across the specimen and causing a detected image to begenerated that has a sampling rate based on the sampling frequencysignal.
 12. The system of claim 11, wherein the chirp clock has a sameperiod as the common trigger clock.
 13. The system of claim 12, whereinthe chirp frequency ramp signal is triggered off an edge of the chirpclock.
 14. The system of claim 12, wherein the chirp frequency rampsignal has a period that is equal to half a period of the chirp clockand the common trigger clock.
 15. The system of claim 11, wherein aperiod of the common trigger clock is selected based on a desired sizeof each spot.
 16. The system of claim 11, wherein a period of the commontrigger clock is selected so that a period of the resulting chirpfrequency ramp signal matches a relative stage movement that causesscanning of the one or more incident spots to move from a first set ofone or more scan lines to a second set of one or more scan lines. 17.The system of claim 11, wherein the pre-scanner clock is delayed fromthe chirp clock by a time duration equal to a fill time of the chirp AODminus a fill time of the pre-scanner AOD.
 18. The system of claim 11,further comprises a diffractive element or mirror system for receiving asingle spot deflected from the chirp AOD and causing a plurality ofspots to be scanned in a plurality of lines across the specimen.
 19. Thesystem of claim 11, wherein each image acquisition clock of eachdetection channel is triggered after a fill time of the chirp AOD. 20.The system of claim 9, wherein a frequency of each acquisition clock isadjusted based on a predefined distortion amount of the correspondingdetection channel.
 21. The system of claim 9, wherein a frequency ofeach acquisition clock is adjusted per each spot of the correspondingdetection channel, wherein such adjustment is based on a predefineddistortion amount for such spot.
 22. The system of claim 9, wherein theacquisition clock and associated sampling frequency signal for eachdetection channel is controlled so that sampling a plurality oflocations on the specimen substantially accurately follows the scanningof the one or more incident spots as they traverse along the pluralityof lines.
 23. The system of claim 9, wherein the acquisition clock andassociated sampling frequency signal for each spot of each detectionchannel is controlled so that sampling a plurality of locations on thespecimen follows the scanning of such spot at it scans along a pluralityof lines.